This invention relates to burning of pulverized coal in a furnace and in particular to a method of introducing fuel and air into the furnace.
Pulverized coal has been introduced in the furnaces for combustion in many different ways including tangential introduction into a furnace with flow upwardly or downwardly therefrom, and vertical firing from the top of the furnace with a reversal of the combustion gases in the furnace and exit through the top of the furnace. Air also has been added to the furnace to support combustion in a plurality of ways. In most cases the air has been supplied in an effort to burn the fuel to the fullest extent possible immediately. In other cases, staged combustion has been used wherein a deficiency of air is supplied with the coal in order to restrict the initial combustion rate.
Oxides of nitrogen, conventionally termed NO.sub.x, are objectionable atmospheric pollutants which occur in combustion products. Two sources of NO.sub.x have been identified. Nitrogen in the air breaks down at high temperature and tends to recombine with oxygen. Nitrogen in the fuel is released even with combustion at lower temperatures, and this also combines with oxygen in the air. The invention relates to its effectiveness in the reduction of the NO.sub.x due to nitrogen in the fuel. By restricting the initial combustion rate and inherently cooling the flame by the furnace walls, the time of combustion is delayed and lower maximum combustion temperatures are achieved. This reduces the formation of oxides of nitrogen which are due to the nitrogen contained in the air.
It does not, however, appear to have a significant effect on the oxides of nitrogen which are formed because of the nitrogen in the fuel.
In the prior art combustion processes the procedure was such that the volatiles which were driven off from the coal by the initial heating continued to flow with the coal. Therefore, as the newly introduced coal mixed with oxygen the volatiles would tend to be the first ones burned. The remaining carbon particles which are more difficult to burn must be oxidized in air which is already partially depleted of oxygen.